Something that's gummy is soft and very sticky. Think of gummy bears, the candy, and you'll get the idea.
Nothing is more gummy than actual gum, especially when it's been chewed and dropped on the sidewalk and then stepped on. Glue is gummy too, and once you've chewed them a little bit, so are Gummy Bears candy. Gummy comes from the noun gum, with its Greek root word, kommi, "gum" or "resin."